This invention relates generally to injection molding of hollow parts and particularly to injection molding of intake manifolds for internal combustion engines. Blow molding of plastic materials to form hollow, thin-walled objects, such as bottles and other containers is well known in the art. The technique involves expanding a gas or air within a parison of plastic material which is expanded outwardly against mold walls of suitable configuration.
The prior art also teaches the manufacture of hollow plastic articles by forming a hollow shell of appropriate configuration, reinforcing the hollow shell, and supporting the hollow shell on a mandrel within a mold cavity configured to provide a space between the exterior of the hollow shell and the interior of the mold cavity. Reinforcement of the hollow shell is accomplished by filling it with water and freezing it to form ice to withstand the forces of the molding process. The ice is melted and the water removed to complete the hollow molded part. Although the art claims that the hollow part may be injection molded, no commercial use of such a process is known to the inventor.
The techniques disclosed in the prior art may be adequate for the injection molding of small simple parts. The techniques do not appear adequate for injection molding a manifold for an internal combustion engine, as is taught by the method of the invention. As is well known, the manufacturers of both large automotive type engines and of small engines for lawn mowers, tools and the like have been seeking a cost effective way to mold intake manifolds without visible success. Because of the high temperatures encountered with internal combustion engines, the plastic molding material must be of a type that requires both high pressure and high temperature. Also the mold itself needs to be maintained in a heated condition to preclude the injected plastic from cooling too quickly. Various lost-metal processes have been used, but none on a commercial basis. The technique suggested by the art, i.e. using an ice filled thin-walled shell has not led to a commercial product either.
The blow-molded hollow shell is fabricated from a different type of plastic material that may experience difficulty in withstanding the above-mentioned high temperatures and pressures, both in the molding process and in operation as a manifold on an engine. In accordance with the invention, a method of manufacturing a manifold for an internal combustion engine involves providing mandrels with grip surfaces for supporting the preformed thin hollow shell such that the ice in the hollow shell securely holds the mandrels in place. The grip surfaces also lock the mandrels and help to prevent distortion of the hollow shell from the high pressure forces encountered in the injection molding process. The mandrels are positioned in mounting flange locations of the manifold and define outwardly tapered inner surfaces for the flanges, which helps to release the mandrels from the manifold. The tapered surfaces also provide for better flow conditions in the manifold during use. The hollow shell is formed with lips that are embedded in the injection molded flange portions of the manifold. This construction secures the shell from movement due to molding pressures. The lips are also spaced from the mounting surfaces of the flanges to insulate the liner (shell) from the high temperatures encountered both in the mold and in an operating engine. The external surface of the shell or inner liner may also be provided with protrusions for firmly securing the inner liner to the interior wall of the injection molded manifold. The provision and spacing of the lips, which are embedded in the manifold flanges, are important in an engine environment to resist any tendency of the liner to separate from the interior wall of the manifold during operation. Since there is no natural bond between the liner and the manifold wall, after many cycles of operation and temperature changes, buckling and separation could occur. In this connection, the protrusions that firmly anchor the inner liner to the manifold wall also assist in resisting flexing and delamination due to temperature and pressure changes during engine operation.